Do not use a tap. That cuts more metal. It will make threads oversized. Run a “thread chaser” of appropriate size through. Look up on eBay etc.
They have wire brushes that are shaped like bottle brushes, in various diameters, that you can stick in a drill chuck and use to clean out some threads. They're usually referred to as "Spiral Wire Tube Cleaning Brushes." But make sure you use a wire material that isn't harder than the thread material. If the threads are brass or aluminum then use a brass wire brush. https://pic8.co/sh/pPgtbT.jpg
Thread chaser, gentle tap run through, wire brush, heat to melt loctite and wipe, brake cleaner… all options. Running the correct bolt in and out a few times with solvent or wd40 works too mostly.
A tap is the usual method. Nothing works as well as a tap...
Makes sense.
Chasing threads is common practice for racing engine builders and other people who refurbish things that need to be right. What are you working on?
I'm trying to clean the threads for an intake manifold.
Conglomerate of comments are the way to go, thread chaser, wire brush, acetone. One exception is Ive never heated to remove loctite but I worked a furnace so heating to remove loctite was never a concern. A good pen oil like liquid wrench w/ a thread chaser after a good wire brush acetone clean will fix any female thread that can be fixed that I've met. That said you can certainly make fucked threads work with enough torque and stupidity.
Wire brush and a little acetone?
Vacuum, then compressed air, tiny bristle brush, more compressed air
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